Drop on demand ink jet printing systems eject ink drops from printhead nozzles in response to pressure pulses generated within the printhead by either piezoelectric devices or thermal transducers, such as resistors. The ink drops are ejected toward a recording medium where each ink drop forms a spot on the recording medium. The printheads have a plurality of inkjet ejectors that are fluidly connected at one end to an ink supplying manifold through an ink channel and at another end to an aperture in an aperture plate. The ink drops are ejected through the apertures, which are sometimes called nozzles.
In a typical piezoelectric ink jet printing system, application of an electrical signal to a piezoelectric transducer causes the transducer to expand. This expansion pushes a diaphragm, which is positioned adjacent the transducer, into a pressure chamber filled with ink received from the manifold. The diaphragm movement urges ink out of the pressure chamber to and through the aperture to eject liquid ink drops. The ejected drops, referred to as pixels, land on an image receiving member opposite the printhead to form an ink image. The respective channels from which the ink drops were ejected are refilled by capillary action from an ink manifold.
In some phase change or solid ink printers, known as indirect printers, the image receiving member is a rotating drum or belt coated with a release agent and the ink is a phase change material that is normally solid at room temperature. In these solid ink printers, the ink image is transferred from the rotating image receiving member to a recording medium, such as paper. The transfer is generally conducted in a nip formed by the rotating image receiving member and a rotating pressure roll, which is also called a transfix roller. One or both of the transfix roller and the recording medium may be heated prior to the recording medium entry in the transfixing nip. As a sheet of paper is transported through the nip, the fully formed image is transferred from the image receiving member to the sheet of paper and concurrently fixed thereon. This technique of using heat and pressure at a nip to transfer and fix an image to a recording medium passing through the nip is typically known as “transfixing,” a well-known term in the art, particularly with solid ink technology.
Ink jet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a recording medium. Duplex printing produces an image on each side of a recording medium. In duplex printing, the recording medium passes through the nip for the transfer of a first image onto one side of the recording medium. The medium is then routed on a path that presents the other side of the recording medium to the nip. By passing through the nip again, a second image is transferred to the other side of the medium. When the recording medium passes through the nip the second time, the side on which the first image was transferred is adjacent the transfix roller. Release agent that was transferred to the first side from the image receiving member to the recording medium may now be transferred to the transfix roller. Thus, a duplex print transfers release agent to the transfix roller and multiple duplex prints may cause release agent to accumulate on the transfix roller.
Additional release agent may be applied to the transfix roller if the transfix roller comes into contact with the image receiving member before the recording medium enters the nip. The amount of release agent on the transfix roller may reach a level that enables release agent to be transferred from the transfix roller to the back side of a recording medium while an image is being transfixed to the front side of the recording medium.
When the first side of a duplex print is being made, the back side of the recording medium, which receives the second image, now has release agent on it. The release agent transferred to the back side of the recording medium may interfere with the efficient transfer of ink from the image receiving member to the back side of the recording medium during duplex printing. Consequently, ink may remain on the image receiving member rather than being transferred to the recording medium. This inefficient transfer of ink may subsequently produce an image in which partial or missing pixels are noticeable. This phenomenon is known as image dropout. Additionally, ink remaining on the image receiving member may require the image receiving member to undergo a cleaning cycle. Undesirable transfer of oil to the transfix roller can be exacerbated if the transfix roller significantly contacts the image receiving member.
To aid in the transfer of ink from the image receiving member to the back side of a recording medium, some printers perform the printing process by controlling the timing for the transfix roller movement as well as the speed of the image receiving member to reduce the likelihood that the transfix roller contacts the image receiving member.
In a duplex print mode, the rotation of the image receiving member is halted prior to engaging the transfix roller to the image receiving member. A media sheet is moved between the transfix roller and image receiving member, and the transfix roller loads against the image receiving member with a margin of the media sheet already positioned between the transfix roller and image receiving member to avoid contact between the transfix roller and release agent. Once the media sheet passes between the transfix roller and imaging receiving member, the image receiving member halts again and the transfix roller disengages from the trailing margin of the media sheet without contacting the image receiving member. This operation may be referred to as a “stop and drop or lift” operation referring to the need to stop the rotation of the imaging drum and either drop or lift the transfix roller away from the imaging drum to prevent release agent from transferring to the transfix roller.
The “stop, drop or lift” operation, however, does not operate the image receiving member at its highest speed continuously and therefore, reduces printer throughput during duplex printing operations. Therefore, performing duplex printing in a manner that improves throughput without subjecting image quality to dropout and the like is useful.